1. Field of the Invention
Exemplary embodiments of the invention relate to a liquid crystal display and a method for driving the same. Although the exemplary embodiments of the invention are suitable for a wide scope of applications, they are particularly suitable for preventing direct current (DC) image sticking, flicker, and stains so as to increase the display quality.
2. Discussion of the Related Art
Active matrix type liquid crystal displays display a moving picture using a thin film transistor (TFT) as a switching element. The active matrix type liquid crystal displays have been implemented televisions as well as display devices in portable devices, such as office equipment and computers, because of the thin profile of the active matrix type liquid crystal displays. Accordingly, cathode ray tubes (CRT) are being rapidly replaced by the active matrix type liquid crystal displays.
Because a liquid crystal display shown in FIG. 1 switches a data voltage supplied to liquid crystal cells Clc using a thin film transistor TFT formed in each liquid crystal cell Clc to actively control data, the quality of a moving picture can increase. In FIG. 1, Cst indicates a storage capacitor for holding the data voltage charged to the liquid crystal cell Clc. DL indicates a data line to which the data voltage is supplied. GL indicates a scan line to which a scan voltage is supplied.
The liquid crystal display is driven in an inversion manner in which a polarity of the liquid crystal cells Clc is inverted between the neighboring liquid crystal cells Clc and the polarity is inverted every one frame period, so as to reduce direct current (DC) offset components and to reduce the degradation of a liquid crystal. If the data voltage with a predetermined polarity is dominantly supplied to the liquid crystal cell Clc for a long time, image sticking may occur. The image sticking is called direct current (DC) image sticking because the liquid crystal cells Clc are repeatedly charged to a voltage with the same polarity. For instance, in case the data voltage is supplied to the liquid crystal display in an interlaced manner, the DC image sticking occurs. In the interlaced manner, the data voltage is supplied to the liquid crystal cells of odd-numbered horizontal lines during odd-numbered frame periods, and the data voltage is supplied to the liquid crystal cells of even-numbered horizontal lines during even-numbered frame periods.
FIG. 2 is a waveform diagram showing an example of the data voltage supplied to the liquid crystal cell Clc in the interlaced manner. In FIG. 2, it is assumed that the liquid crystal cells Clc to which the data voltage is supplied are positioned on odd-numbered horizontal lines.
As shown in FIG. 2, a positive polarity data voltage is supplied to the liquid crystal cells Clc during odd-numbered frame periods, and a negative polarity data voltage is supplied to the liquid crystal cells Clc during even-numbered frame periods. In the interlaced manner, a high data voltage of a positive polarity is supplied to the liquid crystal cells Clc of the odd-numbered horizontal lines during only the odd-numbered frame periods. Therefore, as can be seen from a waveform in a box area of FIG. 2, the positive polarity data voltage is supplied more dominantly than the negative polarity data voltage in 4 frame periods, and thus the DC image sticking appears.
FIG. 3 shows a screen of an experimental result of the DC image sticking appearing by interlaced data. If an original image shown in a left side of FIG. 3 is supplied to the liquid crystal display for a certain time in the interlaced manner, the data voltage, whose a polarity changes in each frame period, noticeably changes depending on the odd-numbered frame periods and the even-numbered frame periods as shown in FIG. 2. As a result, if after the supply of the original image, a data voltage with a middle gray level, for example, 127 gray levels is supplied to all the liquid crystal cells Clc of a liquid crystal display panel, the original image is blurrily displayed on the screen as in an image shown in a right side of FIG. 3. The image shown in the right side of FIG. 3 is the DC image sticking.
As another example of the DC image sticking, if the same image is moved or scrolled at a constant speed, voltages of the same polarity are repeatedly accumulated on the liquid crystal cell Clc depending on a relationship between the size of a scrolled picture and a scrolling speed (moving speed). Hence, the DC image sticking may appear. Another example of the DC image sticking is shown in FIG. 4. FIG. 4 shows a screen of an experimental result of the DC image sticking appearing when an oblique line pattern and a character pattern are moved at a certain speed.
The display quality of the liquid crystal display is reduced by a flicker phenomenon as well as the DC image sticking. The flicker phenomenon means a luminances difference that can be periodically observed with the naked eye. Accordingly, the DC image sticking and the flicker phenomenon have to be simultaneously prevented so as to improve the display quality of the liquid crystal display.
Stains may appear on the display screen of the liquid crystal display. If a DC voltage of the same polarity is applied to a liquid crystal layer for a long time, impurity ions in the liquid crystal layer are separated depending on a polarity of the liquid crystal. Further, ions with different polarities are respectively accumulated on a pixel electrode and a common electrode inside the liquid crystal cells. If a DC voltage is applied to the liquid crystal layer for a long time, the amount of accumulated ions increases. Hence, an alignment layer is degraded and alignment characteristics of the liquid crystal are degraded. In other words, the application of the DC voltage to the liquid crystal display for the long time may cause stains on the display screen. The development of a liquid crystal material with a low dielectric constant or a method for improving an alignment material or an alignment method have been attempted so as to solve the stain problem. However, it takes a long time and a heavy expense to develop a material used in the method. The use of the liquid crystal material with the low permittivity may reduce the drive characteristics of the liquid crystal. According to the experimental findings, as the amount of impurities ionized inside the liquid crystal layer increases and an acceleration factor becomes large, a time when the stains are revealed becomes rapider. The acceleration factor may include a temperature, time, DC drive of the liquid crystal, and the like. Accordingly, the stains may worsen at a high temperature or when the DC voltage of the same polarity is applied to the liquid crystal layer for the long time. Because the stains non-uniformly appear between panels manufactured through the same manufacture line, the stain problem cannot be solved only the development of new material or an improvement method of process. A method for suppressing the DC drive of the liquid crystal is effective in solving the stain problem.